1 1/2 cups all-purpose flour, plus more for dusting
1/2 teaspoon baking powder
1/4 teaspoon salt
6 tablespoons ( 3/4 stick) unsalted butter
1/2 cups granulated sugar, plus 2 additional tablespoons
2 tablespoons confectioners' sugar
1 vanilla bean, split, seeds scraped and reserved
Finely grated zest of 1/2 lemon
1 large egg
Fine sanding sugar, for sprinkling
2 1/4 cups confectioners' sugar
7 1/2 teaspoons meringue powder
Preheat oven to 375. Make the cookies: Whisk together flour, baking powder, and salt in a medium bowl; set aside.
Put butter, granulated and confectioners' sugars, vanilla seeds, and zest into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy. Mix in egg. Reduce speed to low. Add flour mixture in 3 batches; mix until combined.
Roll out dough between 2 pieces of floured parchment paper to 1/8 inch thick. Refrigerate until firm, about 45 minutes.
Remove parchment. Using a 3-inch umbrella cutter, cut out shapes. Reroll scraps, and cut out. Space cookies 2 inches apart on parchment-lined baking sheets. Bake until pale golden, 5 to 6 minutes. Transfer cookies to wire racks to cool.
Make the royal icing: Put confectioners' sugar, meringue powder, and 1/4 cup water into the bowl of an electric mixer fitted with the whisk attachment; beat on low speed 10 minutes. Transfer to a pastry bag fitted with a small plain round tip.
6. Decorate each cookie with icing, and then immediately sprinkle with sanding sugar. (Leftover icing can be stored in an airtight container up to 2 days.)